(1) Field of the Invention
The invention relates to solid state image sensors, and, more particularly, to a method to form light guides for an image sensor to eliminate cross talk between pixels.
(2) Description of the Prior Art
Light imaging array devices are used in a wide variety of applications. These devices utilize an array of active, semiconductor devices, such as diodes, to convert images into streams of digital data.
Referring now to FIG. 1, a prior art image sensor device is illustrated in cross section. This device comprises a semiconductor substrate 10 onto which an array of diodes is formed. Each diode comprises, for example, an n-type region 22 in a p-type region 18. Each diode is separated from other diodes in the array by isolation regions 14 to thereby form an array of pixels. The pixels are used to convert incoming light 50 and 54 from a light/image source 46 into electrical signals via the photo-diodes.
Typically, the substrate 10 is protected by a series of dielectric layers 26, 30, and 34. These dielectric layers will transmit the incoming light 50 and 54 to the pixels 58 and 62. Further, these dielectric layers may comprises intermetal dielectric (IMD) layers for the integrated circuit device wherein metal lines 38 and 42 are formed.
Incident light 50 and 54 will strike the surface of the top most dielectric layer 34. This light will then be transmitted through the underlying dielectric layers 30 and 26 down to the underlying pixels. It is a common occurrence for the incident light 50 and 54 to strike the imaging device surface at a variety of angles. For example, part of the light 50 strikes the surface at nearly a perpendicular angle. Another part of the light 54 strikes the surface at a clearly non-perpendicular angle. The light 50 that strikes the surface at a near perpendicular angle is transmitted to the pixel 58 underlying the strike location. This is optimal for image sensing performance. However, light 54 that strikes the surface at a non-perpendicular angle may then be transmitted to a nearby pixel 62 rather than to the pixel 58 directly underlying the strike surface. This effect is called cross talk. During a cross-talk event, light 54 falls on an incorrect pixel diodes 62 rather than the intended pixel diodes 58 due to light scattering. The light scattering problem causes degraded image resolution for black and white sensors or complicated color correction for color sensors.
In some prior art sensor arrays, multiple layers of metal lines 42 and 38 are used to create metal shields. These metal shields are designed to suppress light scattering between adjacent pixels. However, the use of multiple layer, metal lines 38 and 42 requires that the metal lines be isolated, and this limitation causes the pixel size to increase. A light image sensor device with an improved light shield that does not increase the pixel size is a goal of the present invention.
Several prior art inventions relate to imaging arrays. U.S. Pat. No. 6,001,540 to Huang et al describes a CCD-based imaging array. The array forms microlens by a LOCOS process on a polysilicon layer. Light shield structures, comprising a layer of WSi, are formed over CCD structures that surround the photodiodes. U.S. Pat. No. 5,648,874 to Sawaki et al discloses an optical apparatus. The apparatus uses a matrix comprising acryl resin. Light shielding films are formed overlying and underlying the resin. The light shielding films comprise Cr2O3 or black paint. U.S. Pat. No. 6,195,196 B1 to Kimura et al describes a flat panel display apparatus. U.S. Pat. No. 6,020,944 to Hoshi teaches a LCD apparatus. The apparatus uses a light guide member comprising an anisotropic member with a refractive index anisotropy and a non-anisotropic member with no refractive index anisotropy.